Various power converters topologies and control methods are well known. Power converters convert AC or DC input power into AC or DC output power and may provide electrical isolation between input and output connections. A control method is often used to control the output power in some desired way. Depending upon the topology used, the control method usually varies one of the timing parameters of the power converter, for example: pulse width, switching frequency or phase relationships.
A common, prior art, control method is pulse width modulation, used to regulate the output power. The modulator symmetrically controls the "on" duration of a pair of power switching transistors. These transistors alternately pulse equal "on" durations. Power is increased by increasing the durations, until each reach 50%. Advantages of pulse width modulation include simplicity, and the ability to easily reduce the output power to zero, by reducing the pulse widths to zero. This wide (zero to 100% ) power control range is ideal, and is needed to tolerate a wide normal load range, and a wide range of abnormal operating conditions.
The ability to continuously reduce the output power to zero is required to prevent potentially damaging overcurrent, overvoltage or overtemperature conditions. Not all simple control methods will reduce the power to zero. For example, frequency-modulated symmetrical (2 transistor, "dual-ended") resonant power converters continue to produce some output power when in an "off-frequency" minimum-power control condition.
Resonant L-C circuit elements are often part of the power converter. L-C circuits may be intentionally included in the power path of the converter to reduce EMI and switching losses by slowing the rise time of current and voltage. They may also be unavoidably part of the converter when an AC output converter drives a reactive (L or C) load. In some power converters, maximum efficiency is obtained when the L-C resonant frequency matches the converter's switching transistor frequency.
Unfortunately, the advantageous combination of pulse width modulation in a power converter whose switching frequency and L-C resonant frequency are similar, creates an undesirable "cogging" problem. The cogging results from the interaction of the L-C resonant circuit and the switching transistors that are turned off for part of each switching cycle. During the turned-off time, (also known as "dead-time") the L-C resonant circuit is unclamped and allowed to resonate at its natural frequency. The result is a phase error between the L-C frequency and the switching frequency that frequently "jitters" from one switch cycle to the next often at a sub-multiple of the switching frequency. "litter" may produce audible noise, additional device stress and a reduction in efficiency. Additionally, during "dead-time", parasitic resonant L-C elements are allowed to freely resonate or "ring", which may produce electro-magnetic noise at frequencies above the switching frequency. In the prior art, control methods without "dead-time" such as frequency modulation have been used to control output power. Although cogging and ringing problems are avoided, the simplicity and zero-power capability of pulse width modulation are lost.
Switching power transistors are often used in series, in multiples of 2 (for example: half-bridge, full-bridge, three phase bridge). These switching circuits require isolated FET transistor gate drive for the "upper" isolated transistor. Converters commonly use transformers to couple control signals to the "upper" FET switching transistor and to provide DC isolation. One disadvantage of transformers is their variation in output voltage. The transformer secondary voltage, and therefore the FET gate voltage will vary with the on-time, which results in either excessive FET gate voltage at small pulse width, or else inadequate FET gate voltage at large pulse width. Often, additional circuitry is required to regulate the FET gate voltage. In general, because transformers cannot pass DC, they cannot be used to operate FET transistors at or near 100% duty factors and have a limited duty-cycle range. Other disadvantages of transformers are their size (especially at frequencies of 20 kHz and lower), cost, weight and packaging miniaturization difficulties.
Overtemperature protection is used to prevent power converter damage from overheating due to fault, overload or excessive external ambient temperatures. Typically, the power converter is turned off when a temperature limit is reached. When the converter cools, the converter may be allowed to restart and heat up again. Often, the result is an oscillation when the converter turns on and off repeatedly. This oscillation is undesirable and unavoidable with prior art overtemperature protection methods.
In the prior art, off/on switches are often used between the power converter and its power source. If the power converter uses a large value capacitor across its DC input bus, a large surge of current will flow into the capacitor each time the off/on switch is closed. The surge is typically 10 or 20 times normal operating current, and may cause damage to the switch and other components that repeatedly conduct the surge current. The frequent surge may be eliminated if the off/on switch is relocated into the control circuitry. This change unfortunately requires a relatively expensive switch with gold contacts to operate reliably in low voltage and low current control circuits. The ideal solution would eliminate the surge when the off/on switch is closed, and operate the switch at high voltage but low current. This would allow using an inexpensive switch with non-precious contact material such as brass.
Power converters should be tolerant to excessive input voltage occurring from transients such as lightning strikes and power line over-voltage transients. In the prior art, this tolerance is usually provided by safety factors or de-rating of components that are subject to excessive voltage. These safety factors result in more expensive components and result in a compromise that typically limits the allowable overvoltage to about 20%. The high cost of higher voltage rated switching transistors usually dominate in this compromise. It would be expensive to provide a desirable 100% transistor voltage safety factor.
Therefore there is a need in the prior art for a protected power converter that may be controlled from 0% to 100% output power without transformer caused FET transistor gate drive difficulties. It would be preferable if such a converter would not "cog" with L-C resonant circuit elements while retaining the advantages of pulse width modulation. It would also be preferable if overtemperature protection did not result in undesirable off/on thermal cycling. Another need in the prior art is the elimination of repeated abuse from input surge current when the off/on switch is closed. Also, there is a need for increased tolerance to input overvoltage transients without substantially increased cost.